The present invention relates to a drive circuit for illuminating an electroluminescent (EL) device, and is more particularly directed to a small, efficient, low-power-drain drive circuit which can be miniaturizable, e.g., to be contained on a small printed circuit board.
The invention is more specifically concerned with a microcontroller actuated transformerless drive circuit, in which inductor coil(s) are switched on and off to charge up a storage element, e.g., capacitor, stepwise to a voltage that is many times higher than the dc supply voltage, and then the storage element is discharged. The charge and discharge cycle produces an ac drive voltage to power the EL device.
This arrangement is suitable for use with EL lamps and panels for illuminating clock faces, for backlighting of liquid crystal displays, or for many other applications which are to be dc battery driven, or for marine, automotive, or aviation use.
Several transformerless, i.e., induction flyback driven circuits, have been proposed. In Fujita U.S. Pat. No. 5,581,160, a pair of self-oscillating signal generators turn charging and discharging circuits on and off. These are free-running oscillators and create flyback pulses even during the discharge cycle. This renders it impossible to control the output levels of the drive circuit. Kimball U.S. Pat. No. 5,483,503 involves a circuit for powering an EL lamp in an electronic watch, which employs a microprocessor and logic circuit to actuate a push-pull driver circuit. Alessio U.S. Pat. No. 5,172,032 shows an EL driver circuit in which separate timer circuits actuate the charging and discharging switch elements. Other inductor-based EL drive circuits are shown in U.S. Pat. Nos. 4,529,322 and 5,502,357.
Nothing in the prior drive circuits employs a microcontroller, programmed with suitable firmware, for producing increasing stepped high voltage charge and discharging to create an optimal ac drive waveform. The prior drive circuits have not employed ganged or multiple-inductor charge stages, actuated alternately or sequentially, to increase the rate of charge of the storage capacitor. Also, prior drive circuits have not employed any sort of energy conservation or recharge means to dump current, on the discharge cycle, back into the battery or other dc power source.